1. Field of the Invention
The present invention relates to a method and device for detecting the presence and type of coating on the surfaces of transparent sheet material, and measuring the thickness of transparent sheet material using optical techniques without contacting the transparent sheet material.
2. Description of the Related Art
Transparent sheet materials include glass and plastics and are used for the manufacture of windows, photovoltaic solar panels, displays and a variety of other products. The clear sheet material is often modified by adding color to the raw material or by applying coatings to one or more surfaces of the sheet material. The sheet material is modified to change the material properties for a variety of reasons including architectural appearance, solar control properties, electrical properties, mechanical properties, photochromic properties, electrochromic properties, and others property changes. The final transparency of the sheet material after modification ranges from less than 10 percent to greater than 99 percent of visible light. Many coatings are applied to provide properties relating to wavelengths outside the visible spectrum such as low emissivity coatings which reflect large amounts of the infrared spectrum and thereby provide solar control properties.
The thickness of and presence of coatings on the sheet material are measured at points during manufacture of the sheet material and during secondary fabrication of sheet material into end products such as windows, photovoltaic panels or displays. As the material is cut, coated, tempered, laminated, and fabricated in various other processes into windows or other final products, the manufacturer must determine thickness and the presence of coatings to assure proper assembly of the desired final product. Such measurements are often made by humans, are slow to measure, are difficult to perform accurately, and may involve physical contact of an instrument to the sheet material. Human measurement is slow and highly variable. Physical contact of an instrument to the sheet material may cause damage to the material or to coatings on the sheet material. Separate devices are typically required for thickness and coating determination, and more than one device may be required to determine the side of the sheet on which the coating is present.
Of particular interest is the detection of various low emissivity (lowE) coatings broadly used on windows. LowE coatings control the solar spectrum by selectively reflecting the high heat-carrying portion of the infrared (IR) spectrum. Such coatings reflect the IR light back into the building in high latitude climates, and similarly reflect IR light away from the building in low latitude climates. There are a wide variety of coatings used in industry and many are very similar. These coatings have complex reflection profiles which are often non-monotonic. The complex reflection profile can be used to discern the closely related coatings if a high quality measurement of the reflection profile can be obtained.
Prior art methods for measurement of coatings range from the simple to complex. Simple techniques include the emission of light of a single IR wavelength, such as 905 nm. The IR light is reflected from the coating on the sheet and is detected by an IR receiver. This technique has been successfully implemented by LiteSentry Corporation of Minnesota USA since 2000 with the V1 Thickness and Coating sensor. This technique is limited by the reflectivity of the coatings to a single wavelength of light and in its inability to discern between multiple coatings exhibiting varying reflection at the wavelength chosen.
Another simple technique for detecting coatings is disclosed in U.S. Pat. No. 7,652,760 which describes a conductivity sensor for detecting the presence of certain coatings. This technique is limited by the requirement for conductivity of the coating and in its ability to discern between multiple coatings exhibiting varying conductivity.
A complex technique for the measurement of coatings is a spectrometer. A spectrometer measures the optical response of a sheet material and of the coatings on the sheet material by measuring the transmission and reflection of light over a broad spectrum, typically 500 to 1,500 nm range at high resolution, often 1 nm or better. Spectrometers are broadly used to develop and control the application of thin film coatings on sheet material because of their ability to measure the entire reflection profile of a coating to high resolution and accuracy. A spectrometer offers high resolution and accuracy, but is slow and costly and is therefore not appropriate for coating detection in a sheet fabrication process.
Several prior art sources use triangulation of a laser reflection from the surfaces of the sheet material to measure thickness. ISRA of Germany (WO 2005/085751), Keyence of Japan, EDTM of Ohio USA (U.S. Pat. No. 6,683,695) and other companies offer similar devices for measurement of transparent sheet materials using triangulation techniques. Some of the prior art is also capable of discerning coating, but the methods proposed typically use a linear sensor without a lens or imaging element because this makes triangulation a linear relationship. Because these systems do not use a lens or focusing element, they are limited to using one-dimensional (1D) linear sensing arrays as these are the only sensing elements readily available which are large enough to measure a sufficient distance range to be useful for transparent sheet material. Also, because they do not use a lens or focusing element, the light source used must be highly collimated so the reflections from multiple surfaces do not merge. The use of a 1D linear array requires that multiple light sources be very close together making the design of the source complicated and expensive. Also, with the 1D array the light sources must be turned on sequentially or the light from each source would merge, making the measurement slower and less accurate.